Transfer RNA is the most complex biomacromolecule in both structure and function. It can regulate at the level transcription or translation, as well as enzyme activity. tRNA requires appropriate modification for its effective participation in its many interactions. The methylating enzymes, the most frequent modifying agents, are hyperactive in every malignant tumor and in every tumor there are a few altered isoaccepting tRNA's. We intend to continue our studies of the systems which control methylation of tRNA in normal and neoplastic cells. Changes in activity of the tRNA methyltransferases, and in activity of a competing enzyme, glycine methyltransferase, will be followed during tumor induction by a variety of carcinogens. Other enzymes involved in regulation of methyl metabolism, such as S-adenosylmethionine synthetase and S-adenosylhomocysteine hydrolase will also be monitored. The effects of the perturbation of normal cell metabolism on metabolic pools and tRNA structure will also be investigated. The metabolism of methylated purines derived from the turnover of tRNA will be studied. Cancer patients invariably have elevated serum and urinary levels of these compounds. Two such compounds, 1-methylguanine and 7-methylguanine, cause changes in the growth properties of normal cells in culture, including some instances neoplastic transformation, by an as yet unknown mechanism. The mechanism of pathogenesis by these metabolites which stem from the turnover of tRNA will be studied at the molecular level.